Inference of future bog succession trajectory from spatial chronosequence of changing aapa mires

Abstract Climate change‐driven vegetation changes can alter the ecosystem functions of northern peatlands. Several case studies have documented fen‐to‐bog transition (FBT) over recent decades, which can have major implications, as increased bog growth would likely cause cooling feedback. However, studies beyond individual cases are missing to infer if a common trajectory or many alternatives of FBT are in progress. We explored plant community and hydrology patterns during FBT of 23 boreal aapa mire complexes in Finland. We focused on mires where comparisons of historical (1940–1970) and new (2017–2019) aerial photographs indicated an expansion of Sphagnum‐dominated zones. Vegetation plot and water chemistry data were collected from string‐flark fens, transition zones with indications of Sphagnum increase, and bog zones; thus, in a chronosequence with a decadal time span. We ask, is there a common trajectory or many alternatives of FBT in progress, and what are the main characteristics (species and traits) of transitional plant communities? We found a pattern of fen‐bog transitions via an increase in Sphagnum sect. Cuspidata (mainly S. majus and S. balticum), indicating a consistently high water table. Indicators only of transitional communities were scarce (Sphagnum lindbergii), but FBT had apparently facilitated shallow‐rooted aerenchymatous vascular plants, especially Scheuchzeria palustris. Water pH consistently reflected the chronosequence with averages of 4.2, 3.9, and 3.8, from fen to transition and bog zones. Due to weak minerotrophy of string‐flark fens, species richness increased towards bogs, but succession led to reduced beta diversity and homogenization among bog sites. Decadal chronosequence suggested a future fen‐bog transition through a wet phase, instead of a drying trend. Transitional poor‐fen vegetation was characterized by the abundance of Sphagnum lindbergii, S. majus, and Scheuchzeria palustris. Sphagnum mosses likely benefit from longer growing seasons and consistently wet and acidic conditions of aapa mires.

The FBT is well-known from peat stratigraphies: Sphagnum peat in bogs is most often underlain by sedge peat of earlier fen phases, and the transition tends to be relatively short in duration (Hughes & Barber, 2003;Kuhry et al., 1993;Tuittila et al., 2013) and connected to a shift from high to low pH (Gorham & Janssens, 1992).
The general correspondence of the historical FBT phases in peat profiles with vegetation gradient among contemporary mires supports the approach to consider sequences from fen-to-bog sites as spatial chronosequences of the FBT. However, fens are highly variable in vegetation and water chemistry (Tahvanainen, 2004), and the FBT has alternative trajectories (Hughes & Barber, 2004). Thus, interpretation as chronosequence needs to be well tied to recognized processes and pace of changes (Walker et al., 2010). For example, one much-used peatland chronosequence consists of sites along the land-uplift coast in Finland, where the sequence of mires from primary paludification to fen, transition fen, and bog represents a 4000-year chronosequence of bog development (Juottonen et al., 2022;Laine et al., 2021;Rehell & Virtanen, 2016;Tuittila et al., 2013). However, the connection to decadal timescale changes, relevant to the assessment of 21st-century climate change impacts, is uncertain from long-term peatland development studies of peat stratigraphies and chronosequences recognized so far.
Aapa mires comprise the most abundant mire complex type in northern Fennoscandia (Cajander, 1913;Laitinen et al., 2007;Ruuhijärvi, 1960). Aapa mires are also referred to as string-flark mires, as they have characteristic patterning of hummock strings and hollows called "flarks" with sparse or submerged vegetation that form against the slope and water flow (Figure 1). In the broad sense, aapa mires are synonymous with patterned fens, and they are widespread in the boreal regions of North America Vitt et al., 2022;White & Payette, 2016) and Russia (Kutenkov et al., 2022;Yurkovskaya, 2012). Aapa mires are mire complexes with ombrotrophic bog vegetation in the margins, while central areas are characterized by string-flark fen vegetation (Ruuhijärvi, 1960). The margins between the main zones in these mire complexes have fento-bog gradients that are informative of potential FBT in aapa mires.
Several case studies have shown rapid lateral expansion of bog vegetation across these fen-to-bog gradients (Granlund et al., 2022;Kolari et al., 2022), which makes this case a spatial chronosequence of FBT in a time scale of a few decades in aapa mires. Furthermore, the FBT in this case connects to climate warming and the climate zonation of the focal mire types, as the expanding bog vegetation represents a southern element. Warming-related changes can onset FBT in boreal mires Tahvanainen, 2011), providing one potential mechanism to explain the increase of carbon sequestration in northern mires, as projected for recent past and future (Gallego-Sala et al., 2018).
In Europe, aapa mires prevail in northern and middle boreal zones, while raised bogs predominantly occur in southern and hemiboreal zones (Eurola & Vorren, 1980). The occurrence of aapa mires is largely explained by thermal factors, snow cover, and discharge patterns (Heikkinen et al., 2022;Parviainen & Luoto, 2007;Sallinen et al., 2023). According to Ruuhijärvi (1960), aapa mires predominantly occur north of the isocline of effective temperature sum of 1100°C, but this isocline has moved northwards since his classic work (Sallinen et al., 2023). In northern Finland, the effective temperature sum is projected nearly to double from the 1971-2000 average (Ruosteenoja et al., 2011), and thermal summer may lengthen by 30 days by the period 2040-2069 under the RCP4.5 scenario (Ruosteenoja et al., 2020). Thus, the climate conditions suitable for aapa mires, particularly string-flark fen habitats, are soon lost in the southern range margin (Heikkinen et al., 2022), remarkably affecting the hydrology of aapa mires (Sallinen et al., 2023). Given that supplementary mineral input from the catchment and flooding by snowmelt water maintain the minerotrophic fen vegetation and string-flark patterning of aapa mires, the projected decline of peak flow after snowmelt may induce changes in aapa mires.
One mechanism of FBT, as recently recognized in several Finnish aapa mires, is a lateral expansion of Sphagnum-dominated bog margins over central fens (Granlund et al., 2022;Kolari et al., 2022).
This phenomenon differs from the lateral expansion of mire vegetation over surrounding dry mineral soil, i.e., paludification (Bauer et al., 2003;Kuhry & Turunen, 2006), and it can rather be considered as a type of hydroseral development, initiating as infilling of flarks by floating Sphagna and eventually leading to terrestrialization as bog ecosystem (Granlund et al., 2022). This process in aapa mire

T A X O N O M Y C L A S S I F I C A T I O N
Biodiversity ecology, Botany, Community ecology, Ecosystem ecology, Global change ecology, Landscape ecology complexes likely differs from the mid-Holocene FBTs where transitional stages have often been associated with relatively dry poorfen vegetation with Eriophorum vaginatum (Hughes & Barber, 2003;Hughes & Dumayne-Peaty, 2002;Tuittila et al., 2007;Väliranta et al., 2017). The ongoing expansion of Sphagnum bogs in aapa mire complexes provides an excellent opportunity to study vegetation succession along a spatial chronosequence, as we aim at revealing details of the ongoing and potential future succession trajectories.
In this study, we explore how plant community diversity and structure, and distribution of plant species and traits are altered with the FBT in aapa mires. We collected vegetation and water chemistry data from 23 undrained aapa mire complexes in the northern boreal zone of Finland in 2019-2020. Sampling was focused on sites where a priori comparisons of historical (1944-1970) and new (2017-2019) aerial photographs indicated recent lateral expansions of bog zones by an increase in Sphagnum cover at the transition zones between string-flark fens and bog zones. In all study sites, vegetation plot data were collected from (1) the string-flark fen, (2) the transition zone with an indication of Sphagnum increase, and (3) the bog zone, representing the FBT chronosequence phases. In the three spatial zones, plots were recorded as pairs located on two microhabitats: (1) a hummock string and (2) a flark area between strings (lawns in bogs). In a previous study, we used a multiproxy approach including a detailed analysis of dated peat stratigraphies to verify recent FBTs at five out of the 23 sites studied here (Granlund et al., 2022). Therefore, we are confident that our sampling represents a chronosequence of FBT, although we have not included peat analyses in this study. Instead, we aim to amend the picture of vegetation community and hydrological changes during FBT in aapa mires by including more study sites, thus, potentially covering more variation in FBT, some of which may result from various timing of the phenomenon.

| Study area and site selection
The study sites (n = 23) locate in the north and middle boreal zones of Finland (Table 1). All study sites are characteristic aapa mire complexes, with minerotrophic central fen area and ombrotrophic bog vegetation at the mire margins (Ruuhijärvi, 1960). In the northernmost study site (Hämeenjänkä, Kemijärvi), the mean annual temperature (MAT) for the period 1961-2020 was 0.1°C and the average annual F I G U R E 1 (a) An aapa mire complex, showing the characteristic string-flark pattern in the central fen area and a surrounding bog area with hummocks and lawns. The margins between these main zones have fen to bog gradients that are informative of potential fen-bog transitions (FBT) in the present climate conditions. (b) A close-up image of a fen flark, with sparse and partly submerged vegetation dominated by Carex limosa and C. rostrata. Sources of images: Pasi Korpelainen (UAV image) and Teemu Tahvanainen (lower photograph). to be an appropriate tool for studying the expansion of Sphagnum bogs over wet fens (Granlund et al., 2022;Kolari et al., 2022;Tahvanainen, 2011). In black-and-white photographs, Sphagnum mosses appear in whitish tones, in contrast with dark tones of flark fens with sparse or submerged vegetation. In this study, the degree of Sphagnum increase and disturbances in the mire catchments, potentially altering hydrology, were subjectively assessed from historical and new aerial photographs, and scaled from 0 to 3 (Table 1). The degree of Sphagnum increase varied between sites from an extensive lateral expansion of bog zone to minor changes in Sphagnum cover, and in a few cases, aerial photograph comparisons indicated only subtle changes that were difficult to disentangle from seasonal changes. All study sites are undrained, but disturbances in the catchment varied from no effective to strong TA B L E 1 Descriptions of study sites.

No.
Site Note: Disturbances potentially affecting mire hydrology and a degree of Sphagnum increase were subjectively assessed from historical and new aerial photographs and scaled from 0 to 3. Disturbance: 0 = no effective disturbance, may have clear-cutting in the catchment, 1 = minor disturbance, ditching likely not affecting, 2 = weak to moderate disturbances, ditching likely affecting runoff threshold but not incoming water, and 3 = moderate to strong disturbances, ditching likely reducing incoming water. Sphagnum increase: 0 = from no clear increase to only subtle indication, 1 = minor increase in Sphagnum cover, 2 = clear indication of an increase in Sphagnum cover, and 3 = indication of a large-scale increase in Sphagnum cover.

F I G U R E 2
Historical (left column) and new (right column) aerial photographs for five out of 23 study sites, showing the varying degrees of Sphagnum expansion. In the historical photographs, wet fens are dark gray and black, and Sphagnum-dominated carpets and lawns appear in light gray and whitish tones. New images are color-infrared photos, where Sphagnum mosses appear in pale tones of green and yellow, and green vascular plant vegetation appears in red. Field data were collected from string-flark pairs in fen, transition, and bog zones. Pairs of historical and new aerial photographs for each site are provided in Appendix S1. Source of aerial photographs: National Land Survey of Finland (open data).
disturbances, including mainly forestry ditching and clear-cutting.
Pairs of historical and new color-infrared aerial photographs for each site are provided in Appendix S1. Hereinafter, we refer to the area with an indication of Sphagnum increase as a "transition" zone.

| Field sampling
The study sites were visited in the summer of 2019 and 2020. In each study site, the cover (%) of each plant species was estimated in six locations (n = 155), with some exceptions (Table 1). Most often, data were collected from flark-string pairs as triplets of samples from a minerotrophic string-flark fen, a transition zone, and an ombrotrophic bog of the same mire complex. Technically, plot pairs in bogs located in bog lawns and hummocks, but with flarks and strings, we refer to micro-topographical patterns in all mire zones, if not specified otherwise.
Precise locations of pairs within the mire zones were randomly selected in the field. In each location, coverages of plant species were estimated from a phytosociological relevé and from a nested subplot of 0.25 m 2 . The purpose of phytosociological relevés was to catch whole species assemblages, thus, the size of the relevé depended on the area of the micro-topographical pattern. The relevé size varied between 2 and 25 m 2 but was most often 16 m 2 . In relevés, the abundance scale following the Braun-Blanquet approach was used. In subplots, coverages were estimated on a continuous scale from one to 100%.
Water-table depth (WTD) was measured in each plot as a distance from the water level to the top of the bryophyte layer. Water pH was measured in the field with a portable pH/conductivity tester (Consort). A water sample of 50 mL was obtained in each plot for chemical analyses using a 50-cm long perforated plastic pipe well (n = 145), although some bog hummocks had too deep water table for obtaining a sufficient water sample.

| Species distribution patterns
We used indicator species analysis to determine habitat-association (2) discovering whether transitional communities are more alike with fen or bog communities. Within mire zones, a low mean dissimilarity value indicates low variation and a high-value large variation among study sites.

| Plant functional groups and diversity
We explored differences in species richness, Shannon diversity index, and the total cover and number of species of bryophytes and vascular plants. As species with root aerenchyma are common in wet habitats and capable of transporting methane from peat to the atmosphere (Korrensalo et al., 2022), vascular plants were further divided into shallow-and deep-rooted, and aerenchymatous and nonaerenchymatous species, based on the classic work of Metsävainio (1931) (Kassambara, 2020).
The difference in WTD was similarly tested between zones, but flarks and string hummocks were treated separately.

| Species distribution patterns
Among the 66 species, 37 (56%) showed significant habitat associations in the indicator species analysis (Figure 3). Of these, 15 species showed significant (p < .05) associations to single microhabitats, ten species were associated with combinations of two microhabitats, seven species were associated with three microhabitats, four species with four microhabitats, and one species with five microhabitats.
Several strong indicators were found for the fen flarks

| Plant community compositional turnover
The NMDS ordinations of species abundance data generally illus- The pairwise PERMANOVA confirmed that the bryophyte and vascular plant communities of both flarks and strings differed significantly between all mire zones (Table 2). Significant differences were also found when datasets with full species assemblages were used. PERMANOVA was also run separately for the abundance data of aerenchymatous and nonaerenchymatous species of flarks and strings. Pairwise comparisons showed significant differences between all mire zones in each case, except for nonaerenchymatous species of flarks that were not determined due to several empty plots in fens and transition zones.
Within the mire zones, mean Bray-Curtis dissimilarity values were the highest in the fens and the lowest in the bogs (Table 2).
This applied to full species assemblages, and in separate assess-

| Plant functional groups and diversity
In total, 78 species were recorded in the study sites (36 bryophytes, 38 vascular plants, and four lichens). When both flark and string plots were included in ANOVA, significant differences between mire zones were found in overall species richness (p = .035), and the number of all vascular plant species (p = .002), nonaerenchymatous (p < .001), and aerenchymatous species (p = .004; Table 3). Significant differences were also found in the coverages of bryophytes (p = .001), nonaerenchymatous species (p < .001), and shallow-and On flarks, pairwise comparisons showed that overall species richness and a number of vascular plant species were lowest in fens and increased towards bogs ( Figure 5). The number of bryophyte species was also significantly higher in bogs and transition zones compared with fen flarks, although it was generally very low (2.7 species on average). The cover of bryophytes was significantly higher in the transition zones and bogs than in fen flarks, while no significant difference was found between transition zones and bogs.
The number and cover of nonaerenchymatous species were significantly higher in bogs than in fens and transition zones. The cover of shallow-rooted aerenchymatous species was significantly higher in the transition zone compared with both fens and bogs, while no significant differences were found in the cover of deep-rooted species. No significant differences were found either in the Shannon diversity index, the number and cover of aerenchymatous species, and the cover of all vascular plants.
On strings, pairwise comparisons showed significant differences in the number of bryophytes, aerenchymatous, and nonaerenchymatous vascular plant species, and the coverages of nonaerenchymatous and shallow-rooted aerenchymatous species. The number of bryophyte species was significantly higher in fen strings (mean 3.7) compared with bog hummocks (mean 2.8). The number and cover of nonaerenchymous species were significantly higher in transition zones and bogs than in fens, while transition zone did not differ significantly from bogs. The number of aerenchymatous species was significantly lower in bogs than in transition zones and fens, but the cover did not differ significantly between zones. The cover of shallow-rooted species was significantly higher in fen strings than in bog hummocks.
Beta diversity, as defined for each string-flark pair, was significantly higher in fens than in bogs (p < .0024; Figure 6). Along the fen-bog gradient, beta diversity decreased towards bogs, but no significant differences were found either between transition zones and fens (p = .0554) or between transition zones and bogs.

| Water chemistry and water table
Generally, pH and all mineral element concentrations were low in all study sites. The string-flark fen zones had the highest average pH  and Mn (Figure 7). In the transition zones, pH was significantly lower compared with fens (p < .001), and significantly higher than in the bogs (p < .01). In DOC, no significant difference was found between the fen and transition zones, while DOC was significantly higher in the bog than in the fen (p = .021) and the transition zones (p = .028). S. lindbergii does occur in ombrotrophic bogs as well, its pH range is centered around pH 4.4, higher than other bog Sphagna, and it rarely occurs above pH 5 (Johnson et al., 2015;Wojtuń et al., 2013).

Concentrations of Ca
S. lindbergii is known for the rapid colonization of disturbed sites like thermokarst (Markkula & Kuhry, 2020) and peat-cutting pits (Soro et al., 1999). The species assemblage of transitional flarks included many species that were also frequent in fen flarks (Carex limosa, Rhynchospora alba, Sphagnum jensenii, S. majus, and Scheuchzeria palustris) and fewer species that were shared with bog lawns Contrary to our expectation, deep-rooted aerenchymatous sedges were rare in transitional vegetation and we did not find TA B L E 3 Results from ANOVA with mire zone and micro-topographical pattern as fixed factors and site as a random factor (results for sites not shown).  (Metsävainio, 1931), and as a typical k-strategist, it has large but few seeds. Laberge et al. (2015) found that seed germination of S. palustris was higher on Cuspidata carpets (S. cuspidatum) compared with seedbeds of Cladopodiella fluitans, a common liverwort of fen flarks in aapa mires. As they justified, germination may be hindered on compact C. fluitans seedbeds due to the fact that they are difficult for large seeds to penetrate, while S. Cuspidata carpets are more porous and still have high water level, facilitating germination and better root development (Landry et al., 2012). It should be noted that Scheuchzeria rarely reestablishes spontaneously in restoration sites, despite the rapid increase in Sphagnum, because it is usually absent in the landscape when restoration measures initially take place (Haapalehto et al., 2011). In pristine aapa mires, Scheuchzeria is frequently present in fen flarks, allowing its rapid proliferation with expanding Sphagnum carpets. In our dataset, Scheuchzeria was indicative of all wet surfaces, and it differentiated transitional flarks mainly by higher abundance.

Means for zones
The use of spatial chronosequence allowed comparisons of plant community structure with full species assemblages, while in most paleoecological research, information is limited to fewer identifiable species or to the level of plant structural or taxonomical groups. As In a level of habitat and vegetation types, a majority of transitional flark communities can be considered as noncalcareous quaking mires (Chytrý et al., 2020) that correspond to the alliance Scheuchzerion palustris (Peterka et al., 2017).

| Past and future bog succession trajectories
Our findings suggest that in the present climate, bog succession It is well-known that alternative trajectories towards raisedbog development exist. Hughes and Barber (2004) -Peaty, 2002;Tolonen, 1967;Tuittila et al., 2007;Väliranta et al., 2017). In our case, the succession of aapa mires followed the wet scenario of Hughes and Barber (2004), in which a sedge-fen develops directly to a raised-bog lawn, maintaining a can be considered as a type of secondary hydroseral development from a shallow water body to continuous mats of mosses fixed by vascular plant roots, and finally to a bog phase with peat accumulating above the groundwater level (Moore & Bellamy, 1974). This succession pattern has been reported less often from peat profiles (Granlund et al., 2022;Svensson, 1988;Turunen et al., 2002), but similar vegetation development has been found in case of studies by combining remote sensing and field observations Tahvanainen, 2011) and verified as recent changes (post-LIA) by peat stratigraphies (including five out of our 23 study sites) with a high rate of lateral expansion of wet Sphagna, approximately 70 cm per year (Granlund et al., 2022). Similar to our spatial chronosequence approach, Svensson (1988) collected peat cores along a transect from a mud-bottom hollow and S. cuspidatum carpet to S. magellanicum lawn and S. fuscum-S. rubellum bog in a southern Swedish mire, and correspondingly found S. Cuspidata peat above fen peat, topped by S. magellanicum peat.
The layers of S. sect. Cuspidata previously found in bog peat stratigraphies have been associated with a general rise in humidity and lake water levels (Svensson, 1988), and permafrost thaw in arctic regions . In boreal aapa mire complexes, post-LIA and recent warming and longer growing seasons have likely promoted the growth and expansion of Sphagnum mosses in wet flarks (Bengtsson et al., 2021;Granlund et al., 2022;Loisel et al., 2012).
The moisture supply itself is rarely limited in undrained aapa mires that are fed by high snowmelt and groundwater recharge, enabling Sphagnum to take advantage of warming, although this might change in future (Heikkinen et al., 2022;Sallinen et al., 2023). But most importantly, the weakly minerotrophic conditions have allowed the presence of S. sect Cuspidata species in the fen flarks, thus, enabling their increase without delay from dispersal limitations. pH and concentrations of Ca and Mg were generally extremely low in the studied mire complexes, as pH ranged from 3.3 to 4.7 (with one exception of pH of 5.5), and both Ca and Mg concentrations were under 1 mg/L in the fen flarks. As expected, the pH and concentrations of Ca and Mg were higher in the fen flarks than in transitional communities and bogs. Groundwater pH and Ca concentration are relatively low in the whole Fennoscandia due to glacial history and calcium-poor bedrock , and boreal fens with weak minerotrophy and poor mineral buffering capacity (Tahvanainen et al., 2002) are sensitive to acidification, which may increase susceptibility to Sphagnum increase and FBT (Tahvanainen, 2011). Once established, Sphagnum mosses can further lower pH by the production of organic acids and cation exchange (Clymo, 1963(Clymo, , 1964Schweiger & Beierkuhnlein, 2017). In our study sites, DOC concentration was slightly but not significantly elevated in the transition zones, while in bogs DOC was significantly higher than in the fens and transition zones. Increased DOC concentrations likely connect to the lowered pH, as organic acids are mainly responsible for acidity in mire waters (Tahvanainen et al., 2002), but more detailed water chemical analyses and monitoring would be needed to assess the role of water chemistry.

| Diversity patterns along the fen-bog chronosequence
The string-flark patterning is an important part of diversity in aapa mires. Flarks often have sparse vegetation cover, with few specialized species like the carnivorous Utricularia spp. and some aquatic mosses, but high species richness in strings increases diversity. In our study sites, species richness was generally higher in bogs than in fen and transitional communities, the only exception being relatively low richness in bryophytes in bog hummocks. It must be noted that a majority of our study sites were weakly minerotrophic and some had open-water pools, thus; the flark fens did not harbor rich fen vegetation. At sites with higher minerotrophy and rich fen vegetation, the pattern of species richness would likely be the opposite, decreasing towards bogs. In any case, our results suggest that the progressive fen-to-bog succession will likely lead to lowered compositional heterogeneity of plant communities. This is indicated firstly by the significantly higher beta diversity in fens compared with bogs, and secondly by the greater variation in species composition among fen than bog sites, as indicated by the spread in NMDS ordinations and comparison of within-zone mean Bray-Curtis dissimilarities.

Aapa mires were assessed as Least Concern in the European Red
List of Habitats (Janssen et al., 2016), but the assessment was lacking estimation based on quantitative criteria and potential changes along the FBT trajectory could not be assessed. In the Red List of Finnish habitats, southern aapa mire complex types were assessed as near threatened to threatened, mainly affected by the loss of area and quality by ditching (Kontula & Raunio, 2019). Habitat changes due to natural succession in response to climate change are difficult to assess, as recognition of key mechanisms is demanded, and reliable studies are needed to infer the scale of potential changes.
The overgrowth of acidophilic Sphagnum mosses reported here and in several case studies (Granlund et al., 2022;Kolari et al., 2022;Tahvanainen, 2011) can be a common phenomenon that clearly poses a threat to aapa mire habitat types, but studies so far have focused on exploring the phenomenon, while representative data is lacking on what share of aapa mires is affected. The mosaic pattern of flarks and strings, and the aquatic primary production in flarks are important also for other taxonomical groups than plants. Flarks provide feeding grounds and hummock strings nesting places for birds, including many waders. Thus, the flark infilling and FBT may pose a threat to populations of bird species of mire habitats, that have already declined since the end of the 20th century (Fraixedas et al., 2017;Virkkala & Rajasärkkä, 2012).

| Transitional plant communities in the context of climate change and ecosystem carbon balance
Under the current circumstances of global climate change, the fate of mire carbon storage has received particular attention. Most studies have focused on the direct effects of warming-related treatments on greenhouse gas fluxes, while the indirect effects of plant community changes have often been neglected. However, the effect of warming on net ecosystem CO 2 exchange depends on vegetation composition, and the efflux of CH 4 is more strongly controlled by plant communities than by temperature (Ward et al., 2013). Additionally, the effects of climate change on mire vegetation are likely different in different types of fens and bogs (Kokkonen et al., 2019). While S. sect. Cuspidata can increase in aapa mire complexes with sufficient water supply (Granlund et al., 2022;Kolari et al., 2022) and species of low hummocks in rich fens , prolonged warming and possible lengthening of draught periods may, however, cause desiccation damage and decreased carbon uptake, as inferred for bog Sphagna (Nijp et al., 2014;Norby et al., 2019). Supporting our results of progressive succession instead of desiccation damage, modeling results have indicated increasing carbon sequestration in northern peatlands with increased productivity due to lengthening growing season in the near future (Gallego-Sala et al., 2018), as also indicated by patterns of carbon accumulation in peatlands during the last millennium (Charman et al., 2013). Our results indicate that vegetation changes can contribute in the same direction or act as a mechanism to increase productivity. Sphagnum mosses are known to readily benefit from longer growing seasons (Loisel et al., 2012), as they can continue growing in warm autumns, while many vascular plants are constrained by growth phenology.
Increase of transitional S. sect. Cuspidata-Scheuchzeria palustristype vegetation over fen flarks leads to increased accumulation of decay-resistant Sphagnum peat and soil carbon (Granlund et al., 2022;Loisel & Yu, 2013). An increase in Sphagnum mosses can also reduce CH 4 emissions, particularly in rich fens (Zhang et al., 2021), thus, having climate cooling feedback. However, many vascular plants contribute to mire ecosystem carbon cycle by transporting CH 4 from deep anoxic peat to the atmosphere through plant aerenchyma tissues (Lai, 2009), and an increase in aerenchymatous sedges can enhance CH 4 flux in wet flarks (Strack et al., 2006). In our study, the cover of deep-rooted aerenchymatous species did not differ between fen, transitional, and bog communities, although it was significantly higher in strings and bog hummocks than in flarks and bog lawns. The cover of shallow-rooted aerenchymatous species was significantly higher in transitional flarks compared with fen flarks and bog lawns, which resulted mainly from the increase in Scheuchzeria palustris. Dorodnikov et al. (2011) found that the rate of transported CH 4 from peat to the atmosphere was higher in hollows with S. palustris when compared to drier lawns and hummocks characterized by E. vaginatum. In fact, S. palustris has among the highest methane transport rate of boreal fen species, and it can be responsible for almost half of the ecosystem-scale plant transport in boreal fens (Korrensalo et al., 2022).

| CON CLUS IONS
The identification of the decadal time scale chronosequence of succession from fen to bog made it possible to explore details of vegetation change and test hypothetical differences among functional species groups. Bryophyte cover and communities of flarks appeared most responsive to the FBT, while bryophytes in hummock strings and vascular plants, in general, had more overlap across the chronosequence communities. Our study highlights the potential of ongoing and future shifts in wet boreal fens to Sphagnum-dominated bogs through a transitional phase with Sphagnum sect. Cuspidata-Scheuchzeria palustristype vegetation, while maintaining water-saturated conditions. This leads to the infilling of fen flarks, and the development continues with the formation of bog lawns and hummocks. The transition phase is intermediate in species composition and water chemistry between fen and bog types, with few unique characteristics. In this trajectory, an increase in water-table depth results from the vegetation succession, as the moss surface ascends higher, which may lead to increased accumulation of carbon in the progressively developing Sphagnum peat.
Our results suggest that the shift from fen-to-bog vegetation reduces habitat heterogeneity of aapa mires and leads to more homogenous ombrotrophic bog vegetation. Fennoscandian mire complexes are often weakly buffered with low mineral concentrations, making them sensitive to acidification and ombrotrophication. Changes are likely driven by climate warming, as evidenced in recent studies but can also be triggered by hydrological alterations that call for restoration measures to conserve biodiversity. However, careful considerations are in place with restoration planning, as the bog expansion over aapa mires follows a natural succession pathway of mires, and it likely has desirable cooling feedback. Recognizing progressive responses to changing climate and differentiating from degradation is important and timely in northern mire ecosystems.

ACK N OWLED G M ENTS
Antti Sallinen and Max Strandén are thanked for processing the historical and modern aerial photographs. Pasi Korpelainen is thanked for acquiring the UAV image. Lars Granlund, Ville Vesakoski, and Franziska Wolff are thanked for their assistance in fieldwork. This research was funded by the Academy of Finland (SHIFTMIRE, 311655), the Finnish Cultural Foundation, and the Alfred Kordelin Foundation.

CO N FLI C T O F I NTE R E S T S TATE M E NT
We declare no conflict of interest.